1. Field of the Invention
The present invention is directed to a computed tomography apparatus and to a method for image reconstruction therein, and in particular to a computed tomography apparatus operable in a spiral scan mode and a method for image reconstruction from spiral scan attenuation values.
2. Description of the Prior Art
For image reconstruction by means of a computed tomography apparatus operating in a spiral mode, a measuring system, composed of an x-ray source and a radiation receiver, is rotated around the patient, who is lying on a support table, with relative longitudinal motion between the measuring system and the patient on the support table. An x-ray beam emanates from the x-ray source during operation of the computed tomography apparatus and attenuated by the patient, strikes the radiation receiver. A calculating unit evaluates the measured signals supplied by the radiation receiver during operation, which correspond to spiral attenuation values of the examination subject. In reconstruction of images of planar body slices of the examination subject attenuation values of a planar body slice are derived from the spiral attenuation values acquired from revolutions of the measurement system around the examination subject.
In computed tomography, registration of spiral scans i.e., obtaining spiral attenuation values of body slices of an examination subject, has become a standard technique with great significance for practical application (see, for example, Willy A. Kalender, Principles and Performance of Spiral CT, in L. W. Goldman and J. B. fowlkes, editors, MEDICAL CT and ULTRASOUND: Current Technology and Applications, pages 379-410, Advanced Medical Publishing, 1995). As noted above, the registration of spiral attenuation values usually ensues with a radiological measurement system that continuously moves around an examination subject lying on a patient support table, and the patient support table with the examination subject thereon usually moves with a constant and continuous table feed relative to the measurement system, for example in the z-direction of a Cartesian coordinate system, as shown in FIG. 1. FIG. 1 shows a computed tomography system with the aforementioned measurement system and the patient support table. Due to the relative motion of the patient support table relative to the radiological measurement system, a continuous, spiral scan motion of the radiological measurement system around the examination subject is obtained, so that the spiral attenuation values arise at the different z-positions in the radiological exposures. The z-coordinate of a data set of spiral attenuation values thereby characterizes the relative position of the measured slice represented by the spiral attenuation values of the subject. The movement of the patient support table normally ensues substantially at a right angle relative to the measurement plane, that is defined by the radiological measurement system (see FIG. 1).
For example, U.S. Pat. No. 5,473,658 discloses a computed tomography system for conducting a spiral scan wherein a computer, based on an initial image in the plane of a reference projection and an auxiliary image, recursively calculates a new image at the spacing of d/N.sub.2.pi. from the initial image, whereby d is the slice thickness N.sub.2.pi. is the number of projections on the circumferential angle 2.pi.. Only data from the region of a slice thickness d are utilized for each image.
Several important advantages are achieved with spiral computed tomography compared to planar computed tomography. First, a fast scan of a given volume can be accomplished, and second, the position and the spacing of the images of body slices of an examination subject to be reconstructed can be selected independently of the measurement of the spiral attenuation values, i.e. even after the measurement of the spiral attenuation values. As already mentioned, the spiral data in spiral computed tomography arise at different z-positions, but the known reconstruction algorithms for image calculation generally only work with attenuation values that are produced given a constant z-position of the measurement system. Therefore, attenuation values that correspond to a planar body slice of the examination subject must be generated with the assistance of spiral algorithms before the actual image reconstruction from the spiral attenuation values.
Spiral algorithms that were previously developed are either interpolation methods or weighting methods. The interpolation methods calculate attenuation values for a planar data set, generally from spiral attenuation values respectively in front of and behind the desired image plane, with respect to which an image of the corresponding body slice of an examination subject is to be reconstructed. The known weighting methods usually operate by resorting calculating steps from interpolation methods. Spiral algorithms are effective in terms of image quality in the reconstructed image of a planar body slice of an examination subject particularly as a result of their influence on the noise amplitude and on the slice sensitivity profile. The sensitivity profile indicates the contrast with which a subject detail, that is extremely thin in the slice thickness direction, is imaged in a reconstructed image dependent on its position along an axis parallel to the system axis (see FIG. 1, rotational axis A). Whereas, dependent on the spiral algorithm, the noise can be higher or lower in a conventional exposure, the slice sensitivity profile is usually broader than in the case of a conventional planar exposure. Given a slice gating d, FIG. 2 shows the broadening of the slice sensitivity profile E.sub.s compared to the ideal, rectangular slice sensitivity profile E.sub.i, with the relative sensitivity being shown over the position of the measurement system. A x-ray beam 4 emanating from a focal spot 13 is gated onto a detector 15 with the assistance of two diaphragms 14. It is thus clear that, in addition to the slice thickness, the course of the slice sensitivity profile is also required for evaluating the image quality of a CT system. The contribution of the neighboring layer to the reconstructed image of a desired body slice of the examination subject, and the occurrence of partial-volume artifacts in the image (which shall be explained later) due to object details acquired at the edge of the slice, become lower as the edges of the slice sensitivity profiled become steeper. Known methods (see Willy A. Kalender, Principles and Performance of Spiral CT, in L. W. Goldman and J. B. fowlkes, editors, MEDICAL CT and ULTRASOUND: Current Technology and Applications, pages 379-410, Advanced Medical Publishing, 1995) essentially differ on the basis of the spacing of the interpolation partners and the type of interpolation function. In practice, however, these methods have some disadvantages, as a result of which the aforementioned advantages of spiral computer tomography do not take full effect.
When, for example a few high-contrast objects or parts thereof, for example bones, project only partially into the measured slice, a partial-volume artifact arises that causes a modification of the spiral attenuation values of the object part and its environment, and thus the object contour can also be modified. This artifact becomes more frequent as the thickness of the slice used for the measurement becomes broader (thicker). A reduction of the slice width in fact reduces the occurrence of the artifact, but increases the noise amplitude at the same time.
Further, known spiral algorithms have the inherent consequence of non-uniformly modulating the noise amplitude within a reconstructed image as well as given three-dimensional image reconstruction in the z-direction to varying degrees, which can have an extremely disturbing effect when viewing the image, and may cause the physician to make misinterpretations.
Moreover, if one wishes to evaluate parts of a subject volume with different slice thicknesses with known spiral algorithms, additional measurements respectively made with various settings of the diaphragm in front of the x-ray source are required for this purpose, i.e. with different slice gatings d (also see FIG. 2), with the result of increased radiation exposure for the examination subject. This is particularly unpleasant in the calculation of three-dimensional presentations of regions of an examination subject, since it leads to an anisotropy of the resolution.
An attempt has been made to overcome the recited disadvantages of known spiral algorithms by averaging images, but this approach requires such high image calculating times as to make it impractical, and it precludes the determination of immediate images, i.e., offering the calculated images immediately after the end of the measurement event.